Poling Field Dependence of Piezoelectric Properties and Hysteresis Loops of Polarization versus Electric Field in Alkali Niobate Ceramics

Abstract

The DC poling field dependence of piezoelectricity was investigated to evaluate the mechanism of domain alignment in lead-free ceramics of the form of (1-x)(Na,K,Li,Ba)(Nb0.9Ta0.1)O3–xSrZrO3 (x=0–0.07) by comparison with those of Pb(Zr,Ti)O3 (PZT), PbTiO3 (PT), and BaTiO3 (BT) ceramics. Poling was conducted at 150 °C for 30 min while varying the poling field (E) between ±4.0 kV/mm. By increasing x from 0 to 0.07, the relative dielectric constant (εr), electromechanical coupling factor in the planar mode (kp), frequency constant in the kp mode (fcp), and piezoelectric strain constant (d33) vs E plots showed domain clamping at a specific E. E was the coercive field estimated from the DC poling field dependence. The changes in εr, kp, fcp, and d33 with E became smaller at x=0.06–0.07, because of the proximity to the paraelectric phase. The ceramics with x=0, such as PT or BT ceramics, show similar εr, kp, fcp, and d33 vs E plots. The maximum kp (48%) and d33 (307 pC/N) were obtained for x=0.05 with the lowest fcp of 2964 Hz·m, as shown in the εr, kp, fcp, and d33 vs E plots for ceramics such as tetragonal hard PZT ceramics. Since domain alignment in the ceramics was accompanied by the deformation of crystals as a result of applying the poling field, it was clarified that the lead-free ceramics must have a high kp and high d33 to realize a low fcp, which corresponds to a low Young's modulus. In addition, the optimal ceramic composition was obtained in the typical domain-clamping state from the poling field dependence. Furthermore, it was found that a higher kp was realized at a larger remnant polarization and a smaller coercive field in a symmetrical hysteresis loop of polarization vs electric field (P–E hysteresis loop), because of the easy alignment of ferroelectric domains by applying a poling field. The results of the poling field dependence were also supported by results of expansion strain measurement.